Zirconium based alloy and method for making a component for a nuclear fuel assembly with same

ABSTRACT

The invention proposes a zirconium-based alloy also containing, by weight, apart from unavoidable impurities, from 0.02 to 1% of iron having from 0.8% to 2.3% of niobium, less than 2000 ppm of tin, less than 2000 ppm of oxygen, less than 100 ppm of carbon, from 5 to 35 ppm of sulphur and from 0.01% to 0.25% in total of chromium and/or vanadium, the ratio R of the niobium content, less 0.5%, to the iron content, optionally supplemented by the chromium and/or vanadium content.

FIELD OF THE INVENTION

The present invention relates to zirconium-based alloys that are toconstitute nuclear fuel assembly components usable in light-waternuclear reactors, such as nuclear fuel rod claddings or assembly guidetubes, or even flat products, such as grid plates.

The invention may be used, although not exclusively, in the field of themanufacture of cladding tubes for fuel rods intended forpressurized-water reactors in which the risks of corrosion areparticularly high, and also in the field of strip materials used forstructural components of the fuel assemblies of such reactors. Theinvention also proposes a method for making such components.

BACKGROUND INFORMATION

Patent application PCT WO 99/50 854 proposes a zirconium-based alloyalso containing, by weight, apart from unavoidable impurities, from 0.03to 0.25% in total of iron, on the one hand, and of at least one of theelements of the group constituted by chromium and vanadium, on the otherhand, having from 0.8 to 1.3% of niobium, less than 2000 ppm of tin,from 500 to 2000 ppm of oxygen, less than 100 ppm of carbon, from 5 to35 ppm of sulphur and less than 50 ppm of silicon, the ratio of the ironcontent, on the one hand, to the chromium or vanadium content, on theother hand, being from 0.5 to 30.

The invention is based on observations made by the inventors in thecourse of a systematic study of the intermetallic phases and thecrystallographic forms of those phases which appear when the relativecontents of iron and niobium are varied while the contents of tin,sulphur and oxygen are described in the application mentioned above. Itis also based on the observation, made experimentally, that the natureand the crystallographic form of the intermetallic phases containingzirconium, iron and niobium have a major influence on corrosionresistance in various environments.

In particular, it has been found that the presence of compounds Zr (Nb,Fe)₂ having a crystalline structure with a hexagonal lattice, and of thephase βNb substantially improves corrosion in the aqueous medium whichexists in the majority of pressurized-water reactors.

SUMMARY

The present invention aims especially to provide an alloy which enablescomponents to be obtained wherein the composition may be adapted in anoptimum manner to the conditions of use provided for and whosecomposition is not likely to hamper the manufacturing steps excessively.

To that end, the invention proposes, in particular, a zirconium-basedalloy also containing, by weight, apart from unavoidable impurities,from 0.02 to 1% of iron having from 0.8% to 2.3% of niobium, less than2000 ppm of tin, less than 2000 ppm of oxygen, less than 100 ppm ofcarbon, from 5 to 35 ppm of sulphur and from 0.01% to 0.25% in total ofchromium and/or vanadium, the ratio R of the niobium content, less 0.5%,to the iron content, optionally supplemented by the chromium and/orvanadium content, being higher than 2.5. A ratio exceeding 3 may beused. In order to have a particularly high resistance to uniformcorrosion, iron content may not exceed 0.35% as an example.

The choice of the ratio R results from the observation that the phasehaving a hexagonal lattice does not appear until the relation betweenthe content of Fe (and also of Cr and V if they are present) and thecontent of Nb is such that R exceeds a threshold which depends slightlyon the contents of other elements and on the temperature, but is stillhigher than 2.5.

The oxygen content may be controlled in such a manner that it is from1000 to 1600 ppm.

The invention also proposes a method for making a tube according towhich:

-   -   a bar is produced from a zirconium-based alloy also containing,        by weight, apart from unavoidable impurities, from 0.02 to 1% of        iron, from 0.8% to 2.3% of niobium, less than 2000 ppm of tin,        less than 2000 ppm of oxygen, less than 100 ppm of carbon, from        5 to 35 ppm of sulphur and from 0.01% to 0.25% in total of        chromium and/or vanadium, the ratio of the niobium content less        0.5% to the iron content, optionally supplemented by the        chromium and/or vanadium content, being higher than 2.5;    -   the bar is water-quenched after heating at from 1000° C. to        1200° C.;    -   a blank is extruded after heating at a temperature of from        600° C. to 800° C.;    -   the blank is cold-rolled in at least two passes to obtain a        tube, with intermediate thermal treatments at from 560° C. to        620° C.; and    -   a final thermal treatment is carried out at from 560° C. to 620°        C., all of the thermal treatments being carried out in an inert        atmosphere or under vacuum.

The final thermal treatment leaves the tube in the recrystallized state,which promotes creep strength, without modifying the nature of thephases.

With the method described above, the βNb phase precipitates and thehexagonal-lattice intermetallic compound of the type Zr (Nb, Fe, Cr, V)₂co-exist.

The alloy may also be used to produce flat elements. Those elements arealso used in the recrystallized state and can be manufactured by thefollowing sequence: a blank is produced from a zirconium-based alloyalso containing, by weight, in addition to unavoidable impurities, from0.02 to 1% of iron, from 0.8% to 2.3% of niobium, less than 2000 ppm oftin, less than 2000 ppm of oxygen, less than 100 ppm of carbon, from 5to 35 ppm of sulphur and less than 0.25% in total of chromium and/orvanadium, the ratio of the niobium content less 0.5% to the ironcontent, optionally supplemented by the chromium and/or vanadiumcontent, being higher than 2.5,

-   -   the blank is cold-rolled in at least three passes, with        intermediate thermal treatments and a final thermal treatment,    -   one of those intermediate thermal treatments or a preliminary        thermal treatment before the first cold-rolling pass being        effected for a long period of at least 2 hours at a temperature        lower than 600° C., and    -   any thermal treatment following the long treatment and, in        particular, the final recrystallization treatment, being        effected at a temperature lower than 620° C.

The invention also proposes the application of the above alloy to theproduction of components of nuclear reactors operating with pressurizedwater that contains less than 3.5 ppm of lithium.

The existence of the intermetallic compounds, which is due to thepresence of iron in a sufficient quantity, and particularly theexistence of Zr (Nb, Fe)₂, reduces the amount of niobium precipitates inphase β, but also the niobium content of the solid solution and givesgood resistance to uniform corrosion at a temperature of 400° C., whichis representative of the temperature that prevails in reactors. For aFe/Nb ratio lower than 0.25, the βNb phase is hardly present.

The presence of chromium and/or vanadium as a very partial replacementfor iron and/or niobium in the intermetallic precipitates of the type Zr(Nb, Fe, Cr, V)₂ has no marked effect on corrosion at 400° C. Theimproved corrosion resistance at 400° C. is maintained especially if thesum Fe+Cr is at least 0.03%.

To summarize, an alloy of the above type having a use in therecrystallized state to increase its resistance to the bi-axial creep oftubes and the aptitude for the pressing of sheet metal hascharacteristics which are adjustable by regulating the iron/niobiumratio but which are still favorable; in particular, it has a highcorrosion resistance in an aqueous medium at high temperature, theresistance being all the higher if a high iron content is adopted, thisbeing permitted by a high Nb content.

It also has a high creep strength owing to the presence of tin whichremains at a very low content and, owing to doping with oxygen, at acontent lower than 2000 ppm, which then has no harmful effect oncorrosion resistance.

In current reactors, the ranges given below are particularly valuable asa zirconium-based alloy also containing, by weight, apart fromunavoidable impurities: from 1 to 1.8% by weight of niobium, from 0.1 to0.3% by weight of iron, from 0.15 to 0.20% by weight of tin, from 0.01to 0.1% by weight of chromium and/or vanadium, from 1000 to 1600 ppm ofoxygen, less than 100 ppm of carbon and from 5 to 35 ppm of sulphur.

BRIEF DESCRIPTION OF THE DRAWINGS

The above characteristics and others will emerge more clearly on readingthe following description of particular embodiments which are given byway of non-limiting example. The description refers to the drawingswhich accompany it and in which:

FIG. 1 is a ternary diagram showing the intermetallic compounds andmicrostructures which appear for various ranges of composition, in thecase of a content of 0.2% of tin, at a temperature of from 560° C. to620° C.

FIG. 2 illustrates a fraction of the diagram on a large scale.

DETAILED DESCRIPTION

Referring to the figures, the C, Si and O₂ contents of obtained samplesare substantially identical for all of the samples and were lower thanthe maximum values given above. The tin content was 0.2% and the sulphurcontent was 10 ppm.

The samples were manufactured by thermo-metallurgical operations at atemperature not exceeding 620° C., any treatment exceeding that valuebeyond the extrusion operation reducing corrosion resistance at hightemperature.

The ternary diagram in FIG. 1 shows, for Fe/Nb ratios lower thanapproximately 0.3, the existence of a region in which the aZr phase(with the exclusion of the bZr phase which is very detrimental from thepoint of view of corrosion resistance), the bNb phase precipitates andthe intermetallic phase Zr (Nb, Fe)₂, which has a hexagonal structure,co-exist.

The compounds, corresponding to a ratio (Nb—0.5%)/Fe+Cr+V higher than athreshold which is always higher than 2.5, as an example, are used whenthe main phenomenon to be combated is uniform corrosion inhigh-temperature water having a low lithium content.

For a high Fe/Nb ratio, up to a niobium content of the order of 50%,which is higher by more than one order of magnitude than the contentsused, the compound (Zr, Nb)₄Fe₂, which is face-centered cubic, alsoappears.

When the conditions of use make it desirable to have intermetalliccompounds that have only or predominantly a hexagonal structure, it isfound that the result is achieved by adopting a Fe/Nb ratio lower than0.3, while also respecting the relation (Nb—0.5%)/Fe+Cr+V>2.5.

A precise study of the diagram for the low Fe and Nb contents shows thatthe Nb content in solid solution develops with the Fe content, with Nbremaining constant.

As soon as the Fe content exceeds 60-70 ppm for the alloy according tothe present invention, the hexagonal Zr (Nb, Fe)₂ form appears whichsubstitutes the bNb phase for a ratio by weight of Nb/Fe substantiallyequal to 2.3.

There then appears the face-centered cubic compound (Zr, Nb)₄Fe₂,corresponding to a Nb/Fe ratio substantially equal to 0.6.

This cubic phase (Zr, Nb)₄Fe₂ starts to appear for:

-   -   1% Nb from 0.29 to 0.44% Fe    -   1.5% Nb from 0.49 to 0.66% Fe    -   2% Nb beyond 0.78% Fe.

The diagram illustrates that, by simultaneously increasing the contentof Nb and of Fe, a higher density of intermetallics is obtained, whichpromotes corrosion in an aqueous medium.

The following table shows the influence of the increasing iron content,which does not impair uniform corrosion for an alloy with 1% niobium,the other elements having contents such as described above.

Increase in weight in mg/dm² 415° C. vapour, 311 days, Fe % by weight105 bar 0.03 490 0.15 456 0.29 455

1. A zirconium based alloy comprising: zirconium; and in addition tounavoidable impurities, by weight, from 0.02 to 1% iron; from 0.8% to2.3% niobium, less than 2000 ppm tin, less than 2000 ppm oxygen, lessthan 100 ppm carbon, from 5 to 35 ppm sulfur and from 0.01% to 0.25% intotal of at least one of chromium and vanadium, a ratio(Nb—0.5%)/(Fe+Cr+V) being higher than 2.5.
 2. The alloy according toclaim 1, wherein the oxygen is from 1000 to 1600 ppm.
 3. The alloyaccording to claim 1, wherein the niobium is from 1 to 1.8% by weight,the iron is from 0.1 to 0.3% by weight, the tin is from 0.15 to 0.20% byweight, the at least one of chromium and vanadium is from 0.01 to 0.1%by weight, the oxygen is from 1000 to 1600 ppm, the carbon is less than100 ppm and the sulfur is from 5 to 35 ppm.
 4. A tube comprising: atubular arranged zirconium-based alloy wherein the alloy comprises:zirconium; and in addition to unavoidable impurities, by weight, from0.02 to 1% iron; from 0.8 to 2.3% niobium, less than 2000 ppm tin, lessthan 2000 ppm oxygen, less than 100 ppm carbon, from 5 to 35 ppm sulfurand from 0.01% to 0.25% in total of at least one of chromium andvanadium, a ratio of (Nb—0.5%)/(Fe+Cr+V) being higher than 2.5 in arecrystallized state.
 5. A flat product comprising: a flat arrangedzirconium based alloy wherein the alloy comprises: zirconium; and inaddition to unavoidable impurities, by weight, from 0.02 to 1% iron;from 0.8% to 2.3% niobium, less than 2000 ppm tin, less than 2000 ppmoxygen, less than 100 ppm carbon, from 5 to 35 ppm sulfur and from 0.01%to 0.25% in total of at least one of chromium and vanadium, a ratio(Nb—0.5%)/(Fe+Cr+V) being higher than 2.5 in a recrystallized state. 6.A method of using a component comprising: providing the component madeof an alloy comprising: zirconium; and in addition to unavoidableimpurities, by weight, from 0.02 to 1% iron; from 0.8% to 2.3% niobium,less than 2000 ppm tin, less than 2000 ppm oxygen, less than 100 ppmcarbon, from 5 to 35 ppm sulfur and from 0.01% to 0.25% in total of atleast one of chromium and vanadium, a ratio of (Nb—0.5%)/(Fe+Cr+V) beinghigher than 2.5 in a recrystallized state; and utilizing the componentin a pressurized water reactor, wherein water initially contains lessthan 3.5 ppm of lithium.
 7. The alloy according to claim 1, wherein theratio is higher than
 3. 8. The alloy according to claim 1, wherein theiron content does not exceed 0.35%.
 9. A method for making a tube toconstitute at least one of all and an external portion of at least oneof nuclear fuel rod cladding and a guide tube for a nuclear fuelassembly comprising: producing a bar from a zirconium-based alloy alsocontaining by weight apart from unavoidable impurities, by weight, from0.02 to 1% iron; from 0.8% to 2.3% niobium, less than 2000 ppm tin, lessthan 2000 ppm oxygen, less than 100 ppm carbon, from 5 to 35 ppm sulfurand from 0.01% to 0.25% in total of at least one of chromium andvanadium, a ratio (Nb—0.5%)/(Fe+Cr+V) being higher than 2.5;water-quenching the bar after heating at from 1000° C. to 1200° C.;extruding a blank after heating at from 600° C. to 800° C.; cold rollingthe blank in at least two passes to obtain a tube, with intermediatethermal treatments at from 560 C to 620 C; and carrying out a finalthermal treatment at from 560 C to 620 C, all of the thermal treatmentsbeing carried out in at least one of an inert atmosphere and undervacuum.